Grades of air

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JEScholz

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Location
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I just don't log dives
When I took my Nitrox class and again when I took a VIP class, the issue of partial pressure blending came up and there was a lot of emphasis put on making sure that any 02 clean tank was only filled with "super clean" air. I think it was grade "E" but I don't have my notes.

Can someone point me to the specs for the different air grades or tell me what the contamination difference is between "super clean" air and "normal" dive air?
 
here's another nifty link:

http://www.starairsystems.com/Breathing_Air_Standard.cfm

it gives you the standards for the various CGA (and other) air grades

Grade E is actually pretty dirty air ... it's basically the lowest grade that is safe to compress for human consumption ... i think.... anyway, the link will help
 
As Andy said, it's not grade E. I believe grade J is what you want. What I always ask for is hyrdocarbon-free air. After I get the dumb look, I tell them I want the same air they use to PPB. Then the light comes on.
 
"Modified Grade E" is a common term around here. Its not a CGA grade though and most dive shops don't know much about CGA standards anyway. "Modified Grade E" is supposed to have <0.1 mg/m3 oil (I think that's the number? should be 20x less). Really there's not such thing as "oil free" and most of the blending agencies/standards recognize that minor amounts are ok.

4.9 mg/m3 (just below the CGA Grade E limit) really is pretty contaminated. The standard is rather outdated and I wouldn't say inhaling these levels of oil is exactly safe.

Air from a properly maintained compressor should very easily pass CGA E and most should be "non-detect" for oil. Whether the analytical detection limits meet "modified grade E" is variable depending on the lab and analysis method.
 
Thanks Everyone
Andy, The information from the site you suggested was great, but appears to be somewhat dated. (at least from the NFPA perspective) I took all of the tables and consolidated it into a single table. CGA grade "A" is the lowest standard and CGA Grade "N" the most stringent. Surprisingly, the OCA (Oxygen Compatible Air) standard has an acceptable oil level of .1mg/m3 while CGA Grade "N" calls for "NONE". CGA Grade "N" also has standards for Nitrogen Dioxide, Nitric Oxide and Sulfur Dioxide.

Now I know how clean the air I use is!


CGA Grade N O.C.A. (Oxygen Compatible Air IANTD/ ANDI, '93)
Oxygen 19.5-23.5% 20-22%
Carbon Dioxide 500 ppm 1000 ppm
Carbon Monoxide 10 ppm 2 ppm
T.H.C. (Methane) ---- 25 ppm
Water Vapor 67 ppm 67 ppm
Dew Point -50°F -50°F
Oil / Particles None .1 mg/m3
Odor None None
Nitrogen Dioxide 2.5 ppm ----
Nitric Oxide 2.5 ppm ----
Sulfur Dioxide 5 ppm ----
Halogenated Solvents---- ----
Acetylene ---- ----
Nitrous Oxide ---- ----
Hal. Hydrocarbons ---- ----
 
H2Andy:
here's another nifty link:

http://www.starairsystems.com/Breathing_Air_Standard.cfm

it gives you the standards for the various CGA (and other) air grades

Grade E is actually pretty dirty air ... it's basically the lowest grade that is safe to compress for human consumption ... i think.... anyway, the link will help

Grade D is pretty standard minimum for SCBA use (Scuba without the U - above water)
 
oh yeah, thanks

i'd forgotten about SCBA use
 
These data for Grade E air are current:

Carbon Dioxide.........1000 ppm
Carbon Monoxide.......10 ppm
Oil mist....................5 mg/m3
Volatile Hydrocarbon...25 ppm
Water......................24 ppm



My notes:
Dew point at 1 bar: -65F
Dew point at 240bar: 30F
All fractions rendered volume basis (v/v)
Examples of volatile hydrocarbons are methane, toluene, benzene. Most likely sources would be external as also would carbon monoxide.
Last update to CGA G-7.1: 2004.

To get "oil free"-- whatever they call it, the modified E standard calls for 0.1 mg/m3. IMO, most modern compressors, including some portables, can come very close to that. There is a lot of hype about pure air these days. Grade E is inadequate for divers air. Fortunately, no diver's compressor which is maintained and was made by a reputable manufacturer in the last 10 years actually produces that stuff. Case in point, volatiles, CO and oil mist.
 
Not to put too fine a point on it, the data given by "Star" and others is outdated. Fill Express is up to date. The E standard for moisture has recently been lowered to 24 ppm. This reflects the need to require compressed air which doesn't cause dangerous rust and corrosion in air bottles. The other "E" numbers are still too lenient for use in divers' SCUBA. For example, the effective fraction of CO at 200 feet (fsw, yeah) would be 10 ppm X 7 ata = 70 ppm. That is enough to initiate an Excedrin headache.

Pesky
 

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